The use of non-radioactive labels in biochemistry and molecular biology has grown exponentially in recent years. Among the various compounds used as non-radioactive labels, aromatic dyes that produce fluorescent or luminescent signal are especially useful. Notable examples of such compounds include fluorescein, rhodamine, coumarin and cyanine dyes such as Cy3 and Cy5. Composite dyes have also been synthesized by fusing two different dyes together (Lee et al., (1992) Nucl. Acids Res. 20; 2471-2488; Lee et al., U.S. Pat. No. 5,945,526 and Waggoner et al., in U.S. Pat. No. 6,008,373, all of which are hereby incorporated by reference).
Non-radioactive labeling methods were initially developed to attach signal-generating groups onto proteins. This was achieved by modifying labels with chemical groups such that they would be capable of reacting with the amine, thiol, and hydroxyl groups that are naturally present on proteins. Examples of reactive groups that were used for this purpose included activated esters such as N-hydroxysuccinimide esters, isothiocyanates and other compounds. Consequently, when it became desirable to label nucleotides and nucleic acids by non-radioactive means, methods were developed to convert nucleotides and polynucleotides into a form that made them functionally similar to proteins. For instance, U.S. Pat. No. 4,711,955 (incorporated by reference) disclosed the addition of amines to the 8-position of a purine, the 5-position of a pyrimidine and the 7-position of a deazapurine. The same methods that could add a label to the amine group of a protein could now be applied towards these modified nucleotides.
Labeled nucleotides have been used for the synthesis of DNA and RNA probes in many enzymatic methods including terminal transferase labeling, nick translation, random priming, reverse transcription, RNA transcription and primer extension. Labeled phosphoramidite versions of these nucleotides have also been used with automated synthesizers to prepare labeled oligonucleotides. The resulting labeled probes are widely used in such standard procedures as northern blotting, Southern blotting, in situ hybridization, RNAse protection assays, DNA sequencing reactions, DNA and RNA microarray analysis and chromosome painting.
There is an extensive literature on chemical modification of nucleic acids by means of which a signal moiety is directly or indirectly attached to a nucleic acid. Primary concerns of this art have been with regard to which site in a nucleic acid is used for attachment i.e. sugar, base or phosphate analogues and whether these sites are disruptive or non-disruptive (see for instance the disclosures of U.S. Pat. No. 4,711,955 and U.S. Pat. No. 5,241,060; both patents incorporated by reference), the chemistry at the site of attachment that allows linkage to a reactive group or signaling moiety a spacer group usually consisting of a single aromatic group (U.S. Pat. Nos. 4,952,685 and 5,013,831, both hereby incorporated by reference) or a carbon/carbon aliphatic chain to provide distance between the nucleic acid and a reactive group or signaling moiety and a reactive group at the end of the spacer such as an OH, NH, SH or some other group that can allow coupling to a signaling moiety and the nature of the signaling moiety.
More recently, U.S. patent application Ser. No. 10/096/075, filed on Mar. 12, 2002 (incorporated by reference) has disclosed novel labeling reagents that comprise a reactive group capable of creating a carbon-carbon bond between a marker or label and a desirable target molecule. This is in contrast to labeling reagents described previously, which employed protein derived chemistries involving formation of a bond between an amine, sulfhydryl or hydroxyl group and an appropriate reactive group. The presence and nature of the linker arm may also increase the biological or chemical activity of the labeled target molecule. Linker arms that may be used to provide appropriate spacing of signal groups in nucleic acids were also provided in this disclosure.